Previous studies (NIEHS-MB-003) have shown that the process of gene mutation is under genetic control and that both marked quantitative and qualitative differences can be found in such strains (when compared with the standard wild-type strain) with regard to mutation-induction at specific loci. In two excision repair deficient strains, upr-1 and uvs- 2, enhanced mutant recovery was found after UV or X irradiation -- and more recently after MNNG treatment (NIEHS-MB-Inoue's report of this year). Genetic characterization of the UV-induced ad-3B mutants induced in the repair- deficient strains showed that they were qualitatively different from the UV-induced ad-3B mutants in our standard wild-type strain (NIEHS-MB-003). In a two-component heterokaryon, mutations in the ad-3 region result both from point mutation and chromosome deletions; the latter class is haplo-lethal and cannot be recovered in experiments on haploid strains. Various studies with variation in dose-rate with x-rays, photoreaction after UV, etc., have shown that the lesions giving rise to these two classes of mutations are qualitatively different. The objective of the present study is to determine the effect of upr-1 and uvs-2 on the induction of irradiation-induced chromosome deletions in the ad-3 region.